The present invention relates generally to image communication systems, and more specifically to a data compression technique for image signals using orthogonal transform and vector quantization.
Vector quantization is an efficient data compression technique for full-motion image signals. According to the vector quantization coding technique, the amount of coded image information bits increases with the rate of movement of the image. Under extreme cases, the rate of the coded information bits exceeds the data rate of a transmission channel. When this occurs, the transmission is interrupted, causing discontinuities in the reproduction of the original. Vector quantizers proposed in a paper entitled "Discrete cosine transform coding system using gain/shape vector quantizers and its application to color image coding" (Saito, H., Takeo, H., Aizawa, K., Harashima, H., and Miyakawa, H., Picture Coding Symposium '86, April, Tokyo) is intended to solve this problem by reducing the image resolution as a function of the amount of rapidly moving objects since the human eyes exhibit poor perception to rapid movements. The proposed system comprises a discrete cosine transform encoder which performs discrete cosine transform on a predictive, interframe differential image sequence of the block format so that the orthogonally transformed block signals are mapped on different bands of a frequency domain. A plurality of vector quantizers are connected to the output of the encoder to effect vector quantization on the respective frequency bands. However, a substantial amount of hardware is required for implementing orthogonal transformation at the transmitter and for implementing inverse orthogonal transformation at the receiver.